In recent years, there have been adopted rechargeable and dischargeable electric storage devices, such as battery cells (e.g., a lithium-ion battery cell and a nickel-metal hydride battery cell) and capacitors (e.g., an electrical double layer capacitor). As this type of a battery cell, there is known, for example, a battery cell provided with an electrode assembly that includes a positive electrode including a positive-electrode current collector provided with a positive-electrode active material layer on a surface thereof except one end, and a negative electrode including a negative-electrode current collector provided with a negative-electrode active material layer on a surface thereof except one end, the positive electrode and the negative electrode being wound or stacked with a separator interposed therebetween, and one end of each of the positive electrode and the negative electrode protrudes, as active material layer-unformed parts, from a side end of the remaining electrode different in polarity, and having a structure in which the electrode assembly is housed in a case with the protruding direction of the one end oriented sideways.
Incidentally, a distance from an electrode assembly to an external terminal in a battery cell used for high-power applications, such as industrial equipment's power supplies, unmanned transportation vehicles, EVs (electric vehicles), HEVs (hybrid electric vehicles), aircraft, marine vessels, railways, and the like, needs to be shorted in order to reduce resistance. Considering a reduction in the number of manufacturing steps and an improvement in reliability in addition to this requirement, it is most suitable to adopt a structure in which active material layer-unformed parts of the electrode assembly are bundled together and welded to current collecting members. In addition, in order to improve reliability by preventing airtightness failure and the like, an opening of the case need to be limited to one place. Yet additionally, a cylindrical battery cell or a stacked battery cell has a structure in which an electrode assembly and a case are held in close contact with each other, and therefore, the case has no space for an extra electrolytic solution to be reserved in. Thus, the battery cell is liable to liquid shortage during a prolonged period of use.
From the considerations discussed above, a battery cell for long-term use in a high-power application preferably has a rectangular structure in which the active material layer-unformed parts of the electrode assembly are bundled together and welded to current collecting members. However, the battery cell having such a structure has a problem that a performance of taking up or sucking up an electrolytic solution within the battery cell (hereinafter referred to as “electrolytic solution sucking-up performance”) is not high.
Hence, there has been proposed a battery cell based on the concept that an electrolytic solution reserved in the bottom of a case is taken up or sucked up by means of a capillary phenomenon and supplied to between a positive electrode and a negative electrode by sufficiently reducing respective end-to-end spacings at one end of the positive electrode and/or respective end-to-end spacings at one end of the negative electrode, thereby preventing shortage of the electrolytic solution between the positive electrode and the negative electrode due to long-term use and improving the service life of the battery cell (c.f., Paragraphs 0009 and 0011 in Japanese Patent Laid-Open No. 2010-113920 (Patent Document 1).
In the battery cell described in the abovementioned Patent Document 1, however, it is difficult to adjust respective end-to-end spacings at one end of the positive electrode and/or respective end-to-end spacings at one end of the negative electrode to an aimed design value because of the inherent structure of an electrode assembly. It is also difficult to make the respective spacings uniform. Accordingly, in addition to the problem of being inherently not high in the electrolytic solution sucking-up performance, the battery cell described in the Patent Document 1 has the problem that the electrolytic solution sucking-up performance differs from battery cell to battery cell, thus causing an interindividual difference in battery cell life.
Even if it has been possible to adjust the abovementioned spacings to an aimed design value and make the respective spacings uniform at the time of manufacture, the electrode assembly becomes deformed due to an external force, such as vibration or impact, or an internal stress caused by the expansion and contraction of an active material. Consequently, the battery cell described in the Patent Document 1 has the problem that the abovementioned spacings vary over the time and the electrolytic solution sucking-up performance degrades. In the case of the battery cell described in the Patent Document 1, in particular, which adopts a configuration of suspending the electrode assembly by current collecting members, an external force, such as vibration or impact, may in some instances concentrate on a welded part between the electrode assembly and each current collecting member. In such an instance, it is extremely difficult to keep the abovementioned spacings constant.
In addition, in the battery cell described in the Patent Document 1, an upper portion of the electrode assembly and each current collecting member are welded to each other, and the electrode assembly is compressed at the welded part. Accordingly, the abovementioned spacings may not necessarily be maintained up to the upper portion of the electrode assembly. Thus, the battery cell described in the Patent Document 1 may fail to supply the electrolytic solution up to the upper portion of the electrode assembly.
This sort of problem is not limited to battery cells, but is also true for capacitors (e.g., electrical double layer capacitors).